It works fine if you use just one PROVIDED you determine which switch is the first in the series and install the TP-Link in that location. In case you don't know how to do that, this procedure will work. Kill power at breaker and open up both existing switches. Take the hot wire off both switches and separate it from other wires. Make sure it is not touching other stuff and then turn power back on. With a non-contact voltage tester see which wire is hot, only one will be - that is where you want the new switch.
Yes this will work in a four-way application (three switches), that is what I have in my kitchen. I want to install one in an application where there are four switches and I expect it will work but have yet to do that. Just ordered a few more today to do just that.